Syntax doc edits to avoid implying that JSON-LD is not RDF

***[By David Booth](http://lists.w3.org/Archives/Public/public-linked-json/2013Jul/0132.html):***

Attached are the corresponding changes that I see that are needed in the syntax document:
http://json-ld.org/spec/latest/json-ld/

Other editorial issues that I noticed but I did not fix in the attached version:

1. The Abstract begins: "JSON is a useful data serialization and messaging format".  Delete "useful" or perhaps delete the whole sentence.  (Other data serialization and messaging formats are *not* useful?)

2. The Features at Risk section needs to be updated regarding blank nodes as predicates.